1. Technical Field
The present invention relates to a secondary battery and, more particularly to, a secondary battery with a thermal protector capable of cutting off a current flow by sensing its inner temperature.
2. Related Art
Lithium secondary batteries, which are capable of repeatedly charging and discharging, have been rapidly developed in view of their higher operating voltage and higher energy density per unit weight, compared with nickel-cadmium (Ni—Cd) batteries and nickel-metal hydride (Ni-MH) batteries. The lithium secondary batteries can be classified into liquid electrolyte batteries and solid electrolyte batteries according to the electrolyte used. In general, batteries using a liquid electrolyte are referred to as lithium-ion batteries and batteries using a polymeric electrolyte are referred to as lithium polymer batteries.
However, the lithium polymer batteries have some safety problems. In particular, the lithium polymer batteries consist of a carbonic anode, a lithium oxide cathode, and an organic solvent electrolyte. When a lithium ion battery is overcharged, the electrolyte is decomposed at the cathode and the lithium metal is separated at the anode. As a result, the characteristics of the battery may degrade, and the battery may generate excess heat or even combust.
Since the lithium polymer electrolyte is locally overheated during charging and discharging operations, the polymeric electrolyte that is thermally sensitive may dissolve or soften. As a result, the electric current and potential are nonuniform, and a short circuit may occur. Also, there is a risk of fire or explosion.
In general, the mechanism that causes combustion or heat generation in the lithium ion battery can be explained by an internal short circuit and thermal runaway. To eliminate these risks, the lithium ion battery has been equipped with various kinds of protectors.
A secondary battery with such a protector is disclosed in Japanese Patent Laid-open No. 1999-67188 to Tsumamoto et al., entitled LEAD TERMINAL FOR SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY, published on 9 Mar. 1999. In such a secondary battery, an anode lead terminal includes a lead end for a charging circuit (charging lead end), a collector lead end, a lead end for a discharging circuit (discharging lead end) which is diverged from the collector lead end, and a fuse interposed between the charging lead end and the collector lead end. The lead ends and the fuse are disposed between multiple sheets of resin films. The secondary lithium battery includes a battery unit and a base for the battery unit. As is well known, in the battery unit, a cathode sheet, a separator and an anode sheet are wound together.
The charging lead end and the discharging lead end of the anode lead terminal attached to the anode sheet protrude from the battery unit. A cathode lead terminal with a resin film thereon, attached to the cathode sheet, protrudes from the battery unit.
When the secondary battery having the above structure is overcharged generating heat, the temperature of the fuse installed in the anode lead terminal rises. When the temperature of the fuse rises to 130° C., for example, the fuse blows. As a result, the charging lead end is not electrically connected to the collector lead end, and a charging current flow is cut off.
The above-described secondary lithium battery has the following problems. The fuse is sealed in the resin films and inserted into the case, wherein a thickness of the fuse provides a small gap within the resin films. Since the electrolyte is contained in the case, the electrolyte permeates the fuse through the gap and changes the physical characteristics of the fuse. As a result, it is difficult for the fuse to correctly operate at a particular temperature. Therefore, the secondary battery is unreliable.